Hip stem prosthesis

ABSTRACT

A hip prosthesis is provided for insertion into a femur. In one exemplary embodiment, the hip prosthesis includes a stem having a proximal end, a distal end, and a longitudinal axis. This stem may include anterior and posterior locking surfaces which diverge away from the longitudinal axis. A shank portion may extend distally from the anterior and posterior locking surfaces and converge at an angle distally toward the longitudinal axis.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.11/063,030, entitled “HIP STEM PROSTHESIS,” filed Feb. 22, 2005, thedisclosure of which is hereby expressly incorporated by reference hereinin its entirety.

FIELD OF THE INVENTION

The invention relates to orthopaedic implants. In particular, thisinvention relates to hip stem prostheses.

BACKGROUND

Total hip arthroplasty is often used to restore function to a diseasedor injured hip joint. Positions and directions relative to the hip jointmay be described in terms of proximal being nearer the hip joint, distalbeing further from the hip joint, anterior being nearer the front of thebody, posterior being nearer the back of the body, medial being nearerthe centerline of the body, and lateral being further from the centerline of the body. In total hip arthroplasty, the surfaces of the femurand pelvis are cut away and replaced with substitute implants. In atypical case, the implants include a hip stem component, a femoral headcomponent, and an acetabular component.

The femoral bone is prepared by creating an opening down theintramedullary canal into the femoral bone along an axis from a proximalposition at the upper end of the femur toward a distal position at thelower end of the femur. The pelvis is prepared by reaming theacetabulum. The implants may be placed directly in contact with theprepared bone surfaces for bony fixation of the implant. Alternatively,bone cement may be introduced into the prepared canal and acetabulum sothat it hardens around and locks the components in place.

The hip stem component includes a stem portion extending down into theintramedullary canal of the femur and a neck portion extending away fromthe femur to support the femoral head component.

A recent development is the use of minimally invasive surgicaltechniques in which the bone is prepared and the implants insertedthrough small incisions that cause less trauma to surrounding musclesand other soft tissues such that the patients recovery is faster. Suchminimally invasive surgical techniques can be challenging due to thedifficulty in visualizing the surgical cavity and maneuvering theinstruments and implants within the tight confines of the incision.

SUMMARY

The present invention provides a hip prosthesis for insertion into afemur having a proximal end adjacent a hip joint, a distal end adjacenta knee joint, an anterior side, a posterior side, a medial side, alateral side, and an intramedullary canal.

In one aspect of the invention, the hip prosthesis includes a neck and astem defining a longitudinal axis and having a proximal portion, adistal portion, and a transition portion extending between the proximalportion and the distal portion of the stem. The proximal portion of thestem includes an anterior locking surface, a posterior locking surface,and a lateral surface that angles toward the longitudinal axis of thestem in a proximal direction, the anterior locking surface and theposterior locking surface of the proximal portion forming a firstincluded angle of between six degrees and twenty degrees. The transitionportion of the stem has an anterior face and a posterior face thatextend substantially parallel to the longitudinal axis of the stem andthat form a second included angle that is less than the first includedangle.

In another aspect of the invention, the hip prosthesis includes a neckand a stem defining a longitudinal axis and having a proximal portion, adistal portion, and a transition portion extending between the proximalportion and the distal portion of the stem. The proximal portion of thestem includes an anterior locking surface, a posterior locking surface,and a lateral surface, the anterior locking surface and the posteriorlocking surface of the proximal portion forming a first included angleof between six degrees and twenty degrees, the lateral surface of theproximal portion having an inflection point, the lateral surface anglingtoward the longitudinal axis of the stem in both a proximal directionfrom the inflection point and a distal direction from the inflectionpoint. The transition portion of the stem has an anterior face and aposterior face that form a second included angle that is less than thefirst included angle.

In yet another aspect of the invention, the hip prosthesis includes aneck and a stem defining a longitudinal axis and having a proximalportion, a distal portion, and a transition portion extending betweenthe proximal portion and the distal portion of the stem. The proximalportion of the stem includes an anterior locking surface, a posteriorlocking surface, and a lateral surface that angles toward thelongitudinal axis of the stem in a proximal direction, the anteriorlocking surface and the posterior locking surface of the proximalportion forming a first included angle of between six degrees and twentydegrees, the transition portion of the stem having an anterior face anda posterior face that extend parallel to the longitudinal axis of thestem.

BRIEF DESCRIPTION OF THE DRAWINGS

Various examples of the present invention will be discussed withreference to the appended drawings. These drawings depict onlyillustrative examples of the invention and are not to be consideredlimiting of its scope.

FIG. 1 is a perspective view of an illustrative hip stem according tothe present invention;

FIG. 2 is a front elevation view of the hip stem of FIG. 1;

FIG. 3 is a section view taken along line 3-3 of FIG. 2;

FIG. 4 is a is a section view taken along line 4-4 of FIG. 2;

FIG. 5 is a is a section view taken along line 5-5 of FIG. 2;

FIG. 6 is a is a section view taken along line 6-6 of FIG. 2;

FIG. 7 is a is a section view taken along line 7-7 of FIG. 2;

FIG. 8 is a is a section view taken along line 8-8 of FIG. 2;

FIG. 9 is a is a section view taken along line 9-9 of FIG. 2;

FIG. 10 is a is a section view taken along line 10-10 of FIG. 2;

FIG. 11 is a side elevation view of the hip stem of FIG. 1; and

FIG. 12 is a section view taken along line 12-12 of FIG. 11.

DESCRIPTION OF THE ILLUSTRATIVE EXAMPLES

FIGS. 1-12 illustrate an illustrative hip stem 10 having a proximal end12, a distal end 14, and a longitudinal axis 16 therebetween. The stem10 includes anterior, posterior, medial, and lateral sides 18, 20, 22,24. A neck 17 extends at an angle from the proximal end 12 of the stem10 to support a femoral head component (not shown) in articulating withthe pelvis. The present investigators have found that hip stem fixationis enhanced by designing the stem differently in each of three generalportions including a proximal locking portion 40, a transition portion80, and a distal shank 100.

The proximal locking portion 40 advantageously includes anterior andposterior locking surfaces 42, 44 that diverge from the stem axis 16proximally as best seen in FIG. 10. The locking surfaces 42, 44 arepreferably flat planar surfaces that diverge equally from the stem axis16 to provide initial fixation. Each surface 42, 44 forms an angle withthe stem axis of from 3 to 10 degrees; preferably from 5 to 10 degrees;and more preferably 7 degrees. The included angle 46 is therefore from6-20 degrees; preferably from 10-20 degrees; and more preferably 14degrees. At angles greater than 3 degrees, and especially within thepreferred ranges, the locking surfaces 42, 44 form an anteroposteriorwedge that provides rigid initial fixation and which is highlyresistance to subsidence distally into the femur. The locking surfaces42, 44 also result in axial loads on the stem 10 being advantageouslytransferred to the femur with a relatively larger axial component and arelatively smaller radial component than would be the case with smallerangles such that the femur is axially loaded proximally with reducedpotential for femoral fracture. Finally, the locking surfaces 42, 44result in a definitive stopping point as a surgeon is driving the stem10 into a femur which can be felt indicating that the stem 10 is fullyseated. The locking surfaces 42, 44 converge medially 47 as best seen inFIG. 9 to fit the proximal femoral geometry.

The proximal locking portion also advantageously includes a hyperbolicshaped flat relief surface 48 to better avoid stem 10 impingement withthe bone during stem insertion. In particular, the relief surface 48helps to avoid impingement with the greater trochanter. The surface 48is created by angling the proximal portion of the lateral side 24 intoward the stem axis 16. The relief surface 48 transitions from a flatsurface to a tapering cone extending distally downward toward the tip ofthe stem 10. Referring to FIG. 2, the relief surface 48 forms a surfacethat diverges from the stem axis 16 distally to an inflection point 50which is the most lateral point on the lateral surface 24 andcorresponds to the apex of the hyperbolic shaped surface 48. Thus, bymoving the lateral-most point on the lateral surface 24 distally awayfrom the proximal end 12, the present investigators have improved theinsertion characteristics of the stem 10. From the inflection point 50to the distal shank portion, the lateral surface 24 converges toward thestem axis 16 at angle α, which is a 2.5 degree angle (FIG. 12).

The proximal locking portion 40 extends from 10 to 30 percent of theoverall stem length measured between the proximal and distal ends 12, 14of the stem 10. Preferably the proximal locking portion 40 extends from15 to 25 percent. More preferably, the locking portion 40 extendsdownwardly from the proximal end 12 by 20 percent of the stem length.Preferably, the proximal locking portion 40 includes a porous surfacegeometry 60 to promote bony ingrowth for long term fixation. In theillustrative hip stem 10, the porous surface 60 covers the proximallocking portion 40 in a continuous layer over the anterior, posterior,medial, and lateral sides 18, 20, 22, 24. The porous surface 60 extendsdistally into the transition portion 80 on the medial side 22 and wrapspartway over the anterior and posterior sides 18, 20. Preferably, theporous surface includes a tantalum metal porous surface having astructure similar to that of natural trabecular bone. Such a material isdescribed in U.S. Pat. No. 5,282,861 entitled “OPEN CELL TANTALUMSTRUCTURES FOR CANCELLOUS BONE IMPLANTS AND CELL AND TISSUE RECEPTORS”,issued to R. B. Kaplan and assigned to Ultramet. The material isfabricated of tantalum using vapor deposition. This material has beensold by Implex Corporation of Allendale, N.J., under the tradenameHEDROCEL. Zimmer, Inc., with manufacturing facilities in Warsaw, Ind.,sells a line of surgical implants incorporating this trabecular metaltechnology. The trabecular metal consists of interconnecting poresresulting in a structural biomaterial that is 80% porous and whichallows much greater bone ingrowth compared to conventional porouscoatings and much greater shear strength. In addition, the trabecularmetal possesses a high strength-to-weight ratio. The material isproduced by vapor depositing tantalum on an open celled porous matrix.

The neck 17 advantageously includes opposed flat surfaces 19 formed onit between the proximal end 12 of the stem 10 and the free end 21 of theneck 17. The flat surfaces 19 provide clearance for increasedarticulation of the stem relative to an acetabular component (notshown). The flat surfaces 19 further provide an engagement surface foran instrument to grip the neck 17.

The transition portion 80 of the stem 10 advantageously includesanterior and posterior faces 82, 84 to provide clearance between thestem and the cortical bone of the intramedullary canal of the femurduring minimally invasive surgical procedures. In a minimally invasivehip procedure, soft tissues tend to push the stem 10 into the sides ofthe intramedullary canal such that the stem insertion forces are notacting straight down into the canal. These off axis forces can lead tofemoral fractures. The anterior and posterior faces 82, 84 of thepresent invention relieve initial insertion hoop stresses making steminsertion easier and safer.

The anterior face 82 lies between the anterior locking surface 42 andthe shank portion 100 and the posterior face 84 between the posteriorlocking surface 44 and the shank portion 100. The faces 82, 84 furtherlie between the medial and lateral sides 22, 24. The anterior andposterior faces 82, 84 are preferably angled relative to the stem axisat an angle shallower than the angle of the anterior and posteriorlocking surfaces 42, 44 and preferably at an angle shallower than thetaper of the shank portion 100 which will be discussed below. Preferablythe faces 82, 84 are each angled relative to the axis 16 at an angleless than 3 degrees; more preferably the faces 82, 84 are parallel tothe axis 16.

The distal shank portion 100 generally forms a tapering cone having anincluded angle β of 3 degrees, as shown in FIG. 12. However, whilemaintaining the 3 degree taper angle β, the shank 100 advantageouslytransitions from a generally cylindrical cross section as shown in FIG.6 to a polygonal cross section as shown in FIGS. 7 and 8. The polygonalsections include polygonal faces 102 connected by vertices 104. Thefaces 102 provide clearance for the shank 100 so that only the vertices104 contact cortical bone distally to provide stem centralization whileeasing insertion. The vertices 104 form a press fit with the distalintramedullary canal and may be size to bite into the bone slightly toprovide a degree of rotational resistance to the stem 10. The faces 102also provide a space between the shank 100 and the intramedullary canalto relieve pressure from body fluids within the canal to minimize thepossibility of an embolism upon insertion of the stem 10 into the canal.

Although examples of a hip stem prosthesis and its use have beendescribed and illustrated in detail, it is to be understood that thesame is intended by way of illustration and example only and is not tobe taken by way of limitation. Accordingly, variations in andmodifications to the hip stem prosthesis and its use will be apparent tothose of ordinary skill in the art, and the following claims areintended to cover all such modifications and equivalents.

1. A hip prosthesis having a medial side, a lateral side, an anteriorside, and a posterior side, the hip prosthesis comprising: a neck; and astem defining a longitudinal axis and having a proximal portion, adistal portion, and a transition portion extending between the proximalportion and the distal portion of the stem, the proximal portion of thestem including an anterior locking surface, a posterior locking surface,and a lateral surface that angles toward the longitudinal axis of thestem in a proximal direction, the anterior locking surface and theposterior locking surface of the proximal portion forming a firstincluded angle of between six degrees and twenty degrees, the transitionportion of the stem having an anterior face and a posterior face thatextend substantially parallel to the longitudinal axis of the stem andthat form a second included angle that is less than the first includedangle.
 2. The hip prosthesis of claim 1, wherein the first includedangle formed between the anterior locking surface and the posteriorlocking surface of the proximal portion is fourteen degrees.
 3. The hipprosthesis of claim 1, wherein the second included angle formed betweenthe anterior face and the posterior face of the transition portion iszero degrees.
 4. The hip prosthesis of claim 1, wherein at least aportion of the proximal portion of the stem is encircled by a porouslayer.
 5. The hip prosthesis of claim 4, wherein the porous layerincludes tantalum metal.
 6. The hip prosthesis of claim 4, wherein theporous layer is produced by vapor depositing tantalum metal onto an opencelled porous matrix.
 7. The hip prosthesis of claim 4, wherein theporous layer is 80% porous.
 8. The hip prosthesis of claim 1, whereinthe distal portion of the stem has a proximal cross-section defining acircle and a distal cross-section defining a polygon.
 9. The hipprosthesis of claim 1, wherein the distal portion of the stem forms anincluded taper angle of approximately 3 degrees.
 10. The hip prosthesisof claim 1, wherein the lateral surface of the stem includes aninflection point, the lateral surface of the stem angling toward thelongitudinal axis of the stem from the inflection point in the proximaldirection.
 11. A hip prosthesis having a medial side, a lateral side, ananterior side, and a posterior side, the hip prosthesis comprising: aneck; and a stem defining a longitudinal axis and having a proximalportion, a distal portion, and a transition portion extending betweenthe proximal portion and the distal portion of the stem, the proximalportion of the stem including an anterior locking surface, a posteriorlocking surface, and a lateral surface, the anterior locking surface andthe posterior locking surface of the proximal portion forming a firstincluded angle of between six degrees and twenty degrees, the lateralsurface of the proximal portion having an inflection point, the lateralsurface angling toward the longitudinal axis of the stem in both aproximal direction from the inflection point and a distal direction fromthe inflection point, the transition portion of the stem having ananterior face and a posterior face that form a second included anglethat is less than the first included angle.
 12. The hip prosthesis ofclaim 11, wherein the anterior locking surface and the posterior lockingsurface of the proximal portion of the stem converge medially.
 13. Thehip prosthesis of claim 11, wherein the lateral surface of the stemangles toward the longitudinal axis of the stem at a 2.5 degree angle inthe distal direction from the inflection point.
 14. The hip prosthesisof claim 11, wherein the first included angle formed between theanterior locking surface and the posterior locking surface of theproximal portion is fourteen degrees.
 15. The hip prosthesis of claim11, wherein the second included angle formed between the anterior faceand the posterior face of the transition portion is zero degrees. 16.The hip prosthesis of claim 11, wherein the neck includes an anteriorflat surface and a posterior flat surface, the anterior flat surface andthe posterior flat surface dimensioned to mate with a correspondinginsertion tool to rotationally lock the hip prosthesis to the insertiontool.
 17. A hip prosthesis having a medial side, a lateral side, ananterior side, and a posterior side, the hip prosthesis comprising: aneck; and a stem defining a longitudinal axis and having a proximalportion, a distal portion, and a transition portion extending betweenthe proximal portion and the distal portion of the stem, the proximalportion of the stem including an anterior locking surface, a posteriorlocking surface, and a lateral surface that angles toward thelongitudinal axis of the stem in a proximal direction, the anteriorlocking surface and the posterior locking surface of the proximalportion forming a first included angle of between six degrees and twentydegrees, the transition portion of the stem having an anterior face anda posterior face that extend parallel to the longitudinal axis of thestem.
 18. The hip prosthesis of claim 17, wherein the stem has a stemlength and the proximal portion of the stem extends from 10 percent to30 percent of the stem length.
 19. The hip prosthesis of claim 17,wherein the neck is integrally coupled to the stem.
 20. The hipprosthesis of claim 17, wherein the distal portion of the stem forms anincluded taper angle of approximately 3 degrees.